A polymer containing phosphorylcholine groups has high affinity for water, and thus is superior in moisture absorbing property and moisture-holding property. Further, because of its structure similar to a cellular membrane which is constituted of phospholipids, the polymer has excellent properties such as biocompatibility, non-activating ability of complement, lack of adsorbing property of biomolecules and cells, antithrombus property, and effect of stabilizing lipid bilayer. Research and development relating to synthesis and usage of the polymer aiming at development of the bio-related materials utilizing these properties have been actively performed.
However, although the polymer having the phosphrylcholine group has excellent properties, because of its significantly hydrophilic property as a polar substance, it tends to take harmful polar substances and metals which mingle during the process of manufacturing, e.g. polar substances such as amines, alcohols and carboxylic acids contained in or derived from the starting materials thereof or generated by degradation of the formed polymer; transition metals, organic peroxides and azo compounds added as a polymerization initiator; and degradation products thereof. Upon utilizing a purified product of the polymer as a bio-related material, these impurities may affect safety, odor and configuration of the final product, and frequently give a crucial defect for commercialization.
As a purification method for removing the impurities, some attempts of membrane separation methods in a medium containing water, i.e. in a water system, have been made. However, in any case, impurities which show toxicity or stimulation remains in a large amount in the product, and thus practical use of the polymer as a bio-related material is difficult.
For example, an attempt has been made in which polymerization reaction is performed utilizing an oil-soluble polymerization initiator such as azobisisobutyronitrile in a solvent such as ethanol, and impurities is then removed by a membrane separation method such as dialysis in a water-containing medium (Japanese Laid-open Patent Application Nos. 4-304882 and 6-157270). In these methods, the polymerization initiator and the degradation products thereof as the impurities have oil-soluble character, and are thus difficult to be removed by the membrane separation method in a water system. In addition, dinitryl tetramethylsuccinate, a degradation product of azobisisobutyronitrile, is known to have high toxicity. Then, it may be considered to enhance the oil-soluble character of the water-containing medium. But in this case, removal of polar substances becomes difficult.
Alternatively, there is proposed a method in which a radical is generated on water-soluble cellulose by a cerium ion-containing compound, to graft-copolymerize 2-(methacryloyloxy)ethyl-2-(trimethylammonio)ethyl phosphate in an aqueous solution, and the polymer aqueous solution obtained is then purified by dialysis to remove cerium ions (Japanese Laid-open Patent Application No. 5-220218).
However, a polymerization initiator containing metals such as cerium tends to form a complex with the obtained polymer, and is difficult to be removed sufficiently even by membrane separation in the water system. In this application, it is mentioned that cerium ions were not detected in the purified product extracted with water, and therefore was sufficiently removed. However, in fact, the amounts of cerium ions before and after the purification were not evaluated, and, in addition, the amount of monomers containing the phosphorylcholine groups in the aqueous solution sample after graft-polymerization, which was subjected to detection, was merely a slight amount, i.e. about 10 mg. Thus, it is hardly recognized that whether cerium ions were sufficiently removed from the polymer having the phosphorylcholine groups or not was well evaluated. In practice, the removal of cerium ions is not complete.
Therefore, it has not conventionally known an aqueous solution containing a polymer having phosphorylcholine groups which contains very little amount of impurities or does not contain the impurities at all, let alone a method for effective removal of the impurities, such as polar impurities and toxic impurities, from an aqueous solution containing a polymer having the phosphorylcholine groups.